Secondary batteries have been used in various technical fields such as hybrid vehicles, power tools, power assisted bicycles, cellular phones, and personal computers. This is because the secondary batteries such as lithium-ion batteries, nickel-hydrogen batteries, and nickel-cadmium batteries can be used repetitively by being charged. Each of these secondary batteries generally includes a Ni-plated iron-made or stainless steel-made can or an outer sheath material consisting of a laminated film, an electrolytic solution (electrolyte) accommodated in the outer sheath material, positive and negative electrode bands to which an active material is applied, a separator, and positive and negative electrode terminals. In the secondary battery, a laminated body formed by laminating the positive electrode band, the separator, and the negative electrode band in that order is used as a power generating body, and usually, a member formed by winding this laminated body is housed in the outer sheath material and enclosed in a state of being immersed in the electrolytic solution. The positive electrode band and negative electrode band are connected to a positive electrode lead and a negative electrode lead (both of these are collectively referred to as an “electrode lead”) via the positive electrode terminal and the negative electrode terminal, respectively. In the secondary battery, power generation or charging is performed by the giving and taking of electrons performed by the power generating body immersed in the electrolytic solution in the outer sheath material.
In recent years, the performance of equipment using the secondary battery has increased, and the application range thereof has extended. Therefore, further miniaturization, improvement in heavy-current charging/discharging characteristics, and the like of the secondary battery have been required. To achieve these characteristics, the electrode lead has been required to be formed of a thin plate. However, if the electrode lead is formed of a thin plate, the cross-sectional area decreases, and the electrical resistance increases, which poses a problem of increased loss of electrical energy. In particular, in the application in which the heavy-current charging/discharging characteristics are required, Joule heat generation increases when a heavy current is caused to flow, so that there are fears of thermal effect on organic members, degeneration of the electrolyte, and the like.
As the electrode lead, Ni is preferably used from the viewpoint of corrosion resistance against electrolytic solution; however, Ni is a material having high electrical resistance. On the other hand, materials such as copper, aluminum, and silver each have low electrical resistance; however, copper and aluminum are difficult to be subjected to resistance welding, and silver is an expensive element. Therefore, the use of any of these materials increases the production cost.
To solve these problems, Patent Document 1 proposes a technique in which a clad plate formed of Ni—Cu—Ni having high corrosion resistance is applied to a secondary battery.
Patent Document 2 proposes a lead material for battery having a laminated structure of a weld layer consisting of Ni, Ni alloy, or Fe alloy, and a base layer consisting of at least Cu or heat-resisting Cu alloy. Patent Document 2 discloses, as a specific example, a thin lead material for battery having a total thickness of 0.06 to 0.5 mm.
Non Patent Document 1 shows a specific example in which a clad plate formed of Ni—Cu—Ni is used as a battery terminal and an electrode lead (connecting bar).
Patent Document 3 discloses an invention relating to a production method for a clad bar material consisting of copper or copper alloy though the material quality thereof is different. In Patent Document 3, there is described a method in which after a laminated material has been heated at a predetermined temperature, the material is hot-rolled at a working ratio of 60% or more to produce metallic joint at the clad boundary.